EVIDENCE OF A ROLE FOR PHOSPHATIDYLINOSITOL 3-KINASE ACTIVATION IN THE BLOCKING OF APOPTOSIS BY POLYOMAVIRUS MIDDLE T-ANTIGEN

A polyomavirus mutant (315YF) blocked in binding phosphatidylinositol 3-kinase (PI3-kinase) has previously been shown to be partially defici ent in transformation and to induce fewer tumors and with a significan t delay compared to wild-type virus. The role of polyomavirus middle T antigen-activated PI 3-kinase in apoptosis was investigated as a poss ible cause of this behavior. When grown in medium containing 1D-3-deox y-3-fluoro-myo-inositol to block formation of 3'-phosphorylated phosph atidylinositols, F111 rat fibroblasts transformed by wild-type polyoma virus (PyF), but not normal F111 cells, showed a marked loss of viabil ity with evidence of apoptosis. Similarly, treatment with wortmannin, an inhibitor of PI 3-kinase, stimulated apoptosis in PyF cells but not in normal cells. Activation of Akt, a serine/threonine kinase whose a ctivity has been correlated with regulation of apoptosis, was roughly twofold higher in F111 cells transformed by either wild-type virus or mutant 250YS blocked in binding Shc compared to cells transformed by m utant 315YF. In the same cells, levels of apoptosis were inversely cor related with Akt activity. Apoptosis induced by serum withdrawal in Ra t-1 cells expressing a temperature-sensitive p53 was shown to be at le ast partially p53 independent. Expression of either wild-type or 250YS middle T antigen inhibited apoptosis in serum-starved Rat-1 cells at both permissive and restrictive temperatures for p53. Mutant 315YF mid dle T antigen was partially defective for inhibition of apoptosis in t hese cells. The results indicate that unlike other DNA tumor viruses w hich block apoptosis by inactivation of p53, polyomavirus achieves pro tection from apoptotic death through a middle T antigen-PI 3-kinase-Ak t pathway that is at least partially p53 independent.